There are a few ways to calculate prestress in LS-DYNA. You could use explicit dynamic relaxation directly in WB LS-DYNA (no need to do an Ansys Static Structural analysis). This will use the LS-DYNA explicit solver to apply preload.
If you right click on the Dynamic Relaxation object in Mechanical, you will have options to add General preload (gravity, etc.) and bolt pretension.
Dynamic relaxation is an analysis that occurs before time 0 to apply the pre load and calculate prestress. Explicit dynamic relaxation is similar to performing an explicit dynamic analysis with some damping to find the static equilibrium under pre loads before time 0. We call this pseudo time.
Explicit dynamic relaxation is a good option if the preload is highly nonlinear and/or if you have rigid body motion. Using an implicit solver will not work well or not at all for those cases.
You will find more information here: